The U.S. electric utility industry has recently experienced dramatic changes in the fundamental factors which govern costs associated with increasing electric power requirements. As a result, capital costs of system expansion and associated expenses have resulted in increased power costs to the customer. Furthermore, because power generating plants must have sufficient capacity to meet peak consumer demands, the size of the utility and associated costs are substantially larger than are needed to supply average consumer demand.
To help encourage customers to shape their times of maximum consumption toward off-peak hours, and to more equitably distribute the cost of producing electricity among users, many consumers are charged at least in part in accordance with peak power demand in addition to average or actual power consumed. To minimize energy costs, consumers practice voluntary load shedding, whereby non-vital loads are disconnected from the power lines during peak power demand periods. Further, within a program of involuntary load shedding, some utilities monitor consumption, distinguish between vital and non-vital consumer loads, and selectively disconnect or "shed" non-vital loads from the power lines.
One type of load management technique, known as ripple tone injection, involves unidirectionally impressing encoded audiofrequency pulses directly onto the utility power lines, to be decoded at receivers located at the consumer sites, to carry out desired commands. This type of system is, however, impractical for establishing bidirectional communications of data on the utility power lines because the cost of installing transmitters at the consumer sites is too high. Other techniques for controlling load shape involve transmitting pulse code modulated data to receivers at the consumer sites, each preset to respond to particular code signals to carry out commands. In Gurr, U.S. Pat. No. 4,264,960, for example, a hierarchical system involves a programmable microprocessor based central controller in bidirectional communication with a plurality of microprocessor controlled, substation injection units that inject pulse code signals onto the power lines. Remote receiver units at the customer sites respond to substation injection unit commands to control the on and off times of customer loads. Bogacki, U.S. Pat. No. 4,204,194 discloses an automatic meter reading and control system using the utility power lines, wherein data synchronized to 60 Hz. and in the form of 30 bit per second data pulse bursts at different frequencies are impressed onto the lines.
Problems which have tended to reduce the reliability of systems of the above types include the limited bandpass characteristic of utility power lines as well as electrical noise. For bidirectional communication between the consumer sites and the distribution substation, the distribution line including the distribution transformer can be characterized as a low pass filter with a band pass of approximately ten kilohertz. While commercial power line characteristics display frequency bands of increased signal strength at frequencies greater than ten kilohertz, the frequency of occurrence of these bands varies widely from feeder to feeder, is unpredictable, and as a result, is unacceptable for use as a communications link. In addition, ripple or pulse data systems, being narrow band based, tend to be susceptible to electrical noise distributed throughout the electrical spectrum on the power lines.
A primary object of this invention, therefore, is to provide a system for establishing bidirectional communications on utility power lines substantially independent of the bandpass characteristics of the lines.
Another object is to provide a system for providing bidirectional communications on utility power lines while keeping user costs to a minimum.
A further object is to maximize the number of communication channels established between a control center and geographically remote user sites of a utility power line communication system.
A still further object is to provide a bidirectional communications link between a systems operation center and consumer sites of a utility system with a logical hierarchy that enables data to be distributed substantially without geographical limitation.
A further object is to provide a bidirectional communication system within a utility having high tolerance to narrow band noise sources on the lines and power line attenuation as a function of frequency.
A further object is to provide within an electrical utility distribution network a bidirectional communication system which requires relatively low signal power densities, improves system economy and reduces interchannel interference.
An additional object is to provide a bidirectional communication system within an electric utility distribution system that maximizes the number of communication channels that can co-exist simultaneously on the power lines.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein I have shown and described only the preferred embodiments of the invention, simply by way of illustration of the best modes contemplated by me of carrying out my invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.